1. Field of the Invention
The present invention relates to methods of forming photocathodes and more specifically to methods of forming high sensitivity, large area negative electron affinity (NEA) infrared sensitive transmission mode GaAs on AlGaAs photocathode structures where the GaAs serves as a NEA photoemitter and the AlGaAs serves as a passivating window substrate wherein the method of formation incorporates the best features of both liquid and vapor epitaxal methods.
2. Description of the Prior Art.
The present GaAs/AlGaAs structure, in which the GaAs serves as a NEA photoemitter and the AlGaAs serves as a passivating window substrate, exhibits a transmission mode photoresponse far exceeding that of the conventional multialkali photocathodes in both sensitivity and spectral range upon activation, in a vacuum environment, of the GaAs NEA photoemissive layer with cesium and oxygen. This particular photocathode structure shows improved sensitivity over structures where the GaAs is epitaxially grown on either single crystal GaAs.sub.x P.sub.1-x or single crystal insulating materials like sapphire or spinel because the AlGaAs layer matches the GaAs more closely in lattice parameter and thermal expansion coefficient leading to a higher quality GaAs emitting layer. In addition, because of the "matched" condition, the AlGaAs layer can be made to passivate GaAs by properly doping it p-type. This reduces the surface recombination velocity at the GaAs-AlGaAs interface by bending the energy bands upwards which leads to significantly improved transmission mode photosensitivity. An additional feature of using AlGaAs as a matching layer is that the spectral window range can be made quite large which is desirable for broadband operation. Although this structure exhibits outstanding transmission mode sensitivity, its use in imaging devices is currently not practical because of gross non-uniformities and defects in the surface of the GaAs photoemitter layer. These blemishes have deleterious effects on the image quality. In addition, the lack of thickness control in growing the GaAs layer, which has a significant bearing on reproducing the photocathode response, makes this structure unsuitable for volume production. The surface defects and non-uniformities, as well as the lack of thickness control, arise because of the present method of fabrication. Currently, high sensitivity GaAs/AlGaAs photocathodes are fabricated entirely by liquid epitaxial techniques. This means that both the GaAs emitter layer and the AlGaAs window layer are liquid epitaxially grown on a suitable seed crystal like GaAs or GaP. Defects and non-uniformities are present in the grown layers because of the difficulty in controlling the onset of nucleation and the termination of growth over large areas in a liquid epitaxial process. Of particular difficulty is the controlled heteroepitaxial nucleation and growth of layers less than two to three microns thick which is the required thickness range for the GaAs layer. The reproducible blemish free growth of the GaAs photoemitting layer by liquid methods is therefore a major barrier problem to the production of high image quality and high sensitivity GaAs/AlGaAs transmission mode photocathodes.